Allyl carboxylates

The allyl group, a less familiar protective group for carboxylic acids, can be easily removed from esters by treatment with sodium hydrogen telluride. In contrast to the preceding methods, ethanol has been employed as the solvent. 

Cleavage of allyl carboxylates (general procedure) f To a solution of NaHTe (prepared in situ from tellurium powder (1.30 g, 1.0 mmol) and NaBH4 (0.90 g, 2.4 mmol) in EtOH (20 mL) buffered with deoxygenated HOAc (1.2 mL in 5 mL EtOH)) is added the allyl carboxylate (0.005 mol). The mixture is refluxed under N2 for 2 h, then filtered, the filtrate 

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In copper-catalyzed Grignard reactions of allylic carboxylates, high y regioselectivity is observed when copper(I) cyanide is used as catalyst16. 

Although the majority of allylic etherification reactions have primarily utilized allylic carboxylates or carbonates as electrophiles (and occasionally allylic chlorides), the use of allylic alcohols for this transformation would be more desirable from a practical standpoint. Reported strategies involving Pd catalysis include the use of P(OPh)3 as the ligand197 and Ti(OPf)4198 as an additive for the in situ activation of the hydoxyl group (Equation (49)).199... 

The iridium-catalyzed transformation between carboxyhc acid and vinyl acetate [28] or allyl acetate [30] was also promoted to afford vinyl or allyl carboxylates in good yields. 

The 7-lactam 120, which is very reactive, is obtained from the reaction of methyl 2-(2-methoxycarbonylmethyl-ene)-5-methyl-3,6-dihydro-2//-l,3-thiazine-4-carboxylate 119 with oxalyl chloride and in the presence of triethyl-amine (Scheme 5). Subsequent treatment with methanol affords 3,6-dihydro-2//-l,3-thiazine 121 as a mixture of isomers. Similar treatment of the 4-allyl carboxylate analogue with oxalyl chloride/triethylamine yielded the corresponding 7-lactam <1999J(P1)2449>. 

Allylic carboxylation. Diethyl oxomalonate (1) undergoes a thermal ene reaction with mono-, di-, and trisubstituted alkenes at 145 180°. The reaction is also subject to catalysis with Lewis acids, which can lead to a different ene product. The products are a-hydroxymalonic esters. The corresponding malonic acids are converted to carboxylic acids by bisdecarboxylation with NaI04 and a trace of pyridine- or with ceric ammonium nitrate (CAN). Diethyl oxomalonate then functions as an cnophilic equivalent of C02. 

Copper-catalyzed allylic oxidation allows the functionalization of unactivated alkenes into chiral allylic carboxylates.1347 The use of oxazoline-containing ligands give good enantioselectivities, but the reaction is extremely slow.1348-1351 Chiral bipyridine complexes, in turn, are much more active and give products in good yields and enantioselectivities up to 70% when applied in benzoyloxylation of cycloalkenes with rm-butyl perbenzoate.1352,1353... 

Cyclization ofl,6-enynes to 1,3- and 1,4-dienes.1 The 1,6-enynes are prepared by Pd(0)-catalyzed alkylation of allylic carboxylates with the anion of dimethyl propargylmalonate. The products cyclize in the presence of this Pd(II) catalyst or [(o-CH3C6H4)3P]2Pd(OAc)2 to 1,3- or 1,4-dienes. 

An additional example utilizing phosphorus nucleophiles employs lithium diphenylthiophosphides and allyl carboxylates and yields allylic diphenylphosphine sulfides (equation 66).2 1... 

Allyl carboxylates can serve as protecting groups for acid functionalities removable by formate and... 

The reaction of 3-ketoacids with allyl carboxylates is also believed to proceed via a palladium enolate intermediate.126 Less than complete stereospecificity is also observed in these reactions (equation 163). Interestingly, the bicyclic lactone substrate employed to ascertain the stereointegrity of this reaction, in addition to being incapable of any syn-anti isomerization, cannot epimerize the starting material by car-boxylate attack at the metal. The observed stereochemical leakage could be due to epimerization of the intermediate allyl complex (equation 164) or reductive elimination of an allylpalladium enolate (retention) (equation 165). 

Although simple ketones and esters can not be allylated by Pd catalysts, they are allylated with allyl carbonates via their enol ethers of Si and Sn. In the allylation of the silyl enol ether 202 with allyl carbonate 200, transmetallation of 202 with the n-allylpalladium methoxide 201, generated from allyl methyl carbonate (200), takes place to generate the Pd enolates 203 and 204. Depending on the reaction conditions, allyl ketone 205 is formed by the reductive elimination of 203 [100]. When the ratio of Pd Ph3P is small, the a,/i-unsaturated ketone 206 is obtained by -elimination [101]. For example, the silyl enol ether 208 of aldehyde 207 is allylated with allyl carbonate (200) to give a-allylaldehyde 210 via 209. The a-allyl carboxylate 213 is obtained by allylation of ester 211 with allyl carbonate (200), after conversion of ester 211 to the ketene silyl acetal 212 [102], As the silyl group is trapped in these... 

The ajf-disubstituted cyclohexane 414 is prepared by intramolecular allylation of 412 to generate 413, and subsequent decarboxylation-allylation. The diallylation reactions of 412 are based on the fact that intramolecular allylation of the /i-kcto ester 412 with the allylic carbonate moiety is faster than decarboxylation of the allyl carboxylate moiety in 412 [179]. 

Allylic carboxylates also react with gold catalysts. These reactions are formally carbonyl activations, but the products... 

Axial allylic carbonyl groups have only a slight directing effect in peracid epoxidations, the effect of equatorial allylic carboxyl groups is even smaller68,69. 

The enolate formed in this process then condenses irreversibly with the r -allylpalladium species. At the outset, dimedone and A,A-dimethylbarbituric acid had been more specifically devisedP for the deprotection of allyl carbamates in solution or in solid-phaseP chemistry in which case the enolate species also forms by proton transfer from the (3-dicarbonyl allyl scavenger to the liberated amine but since then it has also been used for the deprotection of allyl carboxylic esters (see Section 2.2.1.1.3).P 1... 

The order of reactivity of this Ru/silane combination to various functional groups differs greatly from that of its Pd/silane/ZnCh analog. While the latter is very useful for allylic reductions and essentially useless for unsaturated esters, the Ru-based system exhibits opposite reactivity. This complementary che-moselectivity is illustrated by the reduction of cinnamyl cinnamate (Scheme 59), a substrate containing both an allylic carboxylate and an a, -unsaturated ester.Each of these can be reduced separately by silicon hydride and the appropriate transition metal catalyst. 

Following the building-block approach, artificial glycopeptides were synthesized by Wong et al. [40], as small-molecule mimics of sialyl Lewis . They utilized the fucosylated amino acid (9-allyl carboxylic ester building block (Scheme 14.21). The properly protected, threonine... 

Although nucleophiles generally substitute allylic leaving groups with retention of configuration, the substitution of allylic carboxylates sometimes proceeds in a nonstereospecific manner due to palladium-catalyzed isomerization of the substrates (Scheme 48). ... 

Addition to allenes. Allylic carboxylates and amines are formed in such addition reactions. 

Other allylic compounds having C-0 bonds, such as allylic carboxylates, carbonates, phosphates, ethers, and alcohols, can also serve as a substrate in such oxidative addition reactions via the C-0 bond cleavage [1]. 

An r 3-allylruthenium(II) complex has been isolated by oxidative addition of allyl carboxylate to Ru(0) complex (Eq. 9) [15]. 

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